Atherosclerotic occlusive disease is the primary cause of stroke, heart attack, limb loss, and death in the US and the industrialized world. Atherosclerotic plaque forms a hard layer along the wall of an artery and is comprised of calcium, cholesterol, compacted thrombus and cellular debris. As the atherosclerotic disease progresses, the blood supply intended to pass through a specific blood vessel is diminished or even prevented by the occlusive process. One of the most widely utilized methods of treating clinically significant atherosclerotic plaque is balloon angioplasty.
Balloon angioplasty is an accepted and common method of opening blocked or narrowed blood vessels in every vascular bed in the body. Balloon angioplasty is performed with a balloon angioplasty catheter. The balloon angioplasty catheter consists of a cigar shaped, cylindrical balloon attached to a catheter. The balloon angioplasty catheter is placed into the artery from a remote access site that is created either percutaneously or through open exposure of the artery. The catheter is passed along the inside of the blood vessel over a wire that guides the way of the catheter. The portion of the catheter with the balloon attached is placed at the location of the atherosclerotic plaque that requires treatment. The balloon is inflated to a size that is consistent with the original diameter of the artery prior to developing occlusive disease.
When the balloon is inflated, the plaque is stretched, compressed, fractured, or broken, depending on its composition, location, and the amount of pressure exerted by the balloon. The plaque is heterogeneous and may be soft in some areas or hard in others causing unpredictable cleavage planes to form under standard balloon angioplasty. The basic mechanism of balloon angioplasty relies to on a combination of actions caused by the balloon exerting pressure on the atherosclerotic plaque, including; compression of the plaque and the fracture of the hard, circumferentially calcified portion of the plaque. Balloon angioplasty causes plaque disruption and sometimes it causes arterial injury at the angioplasty site. Balloon angioplasty is often performed at high inflation pressures, in excess of 4 atmospheres, very commonly at 8 atm and sometimes up to 22 atm. Therefore, the results of balloon angioplasty are unpredictable.
When the angioplasty balloon is expanded with enough pressure to open a hard plaque dissection often occurs; the hardened areas become disrupted and partially separated from the arterial wall and are prone to lifting up as flaps or chunks. The higher the pressure of balloon angioplasty and the more rapidly the pressure reaches a high level, the more often it produces dissection. The random cleavage planes that are created by the dissection depend upon the composition of the plaque and the pressure exerted upon it. The cleavage planes tend to be wandering, longitudinal lines. The depth of the cleavage planes or fractures that are created by balloon angioplasty varies significantly and may be superficial or may be deep and extend all the way to the media of the arterial wall. To the extent that the cleavage plane goes across the line of flow, that is perpendicular or diagonal to the axial direction of the vessel, there is the potential for partial or complete lifting of a flap. When a flap of fractured plaque has lifted, it may cause acute occlusion or blockage of blood flow, or leave a significant residual stenosis, or may extend to create a larger flap.
Frequently, a segment of the plaque is more resistant to dilatation than the remainder of the plaque. When this occurs, greater pressure pumped into the balloon results in full dilatation of the balloon to its intended size. The balloon is deflated and removed and the artery segment is reexamined, usually using angiography. The process of balloon angioplasty is one of uncontrolled plaque disruption. The lumen of the blood vessel at the site of treatment is usually somewhat larger, but not always and not reliably. Some of the cleavage planes created by fracture of the plaque with balloon angioplasty form dissection. A dissection occurs when a portion of the plaque is lifted away from the artery and is not fully adherent and may be mobile or loose. The plaque that has been disrupted by dissection protrudes into the flowstream. If the plaque lifts completely in the direction of blood flow, it may impede flow or cause acute occlusion of the blood vessel.
The dissection of plaque after balloon angioplasty is treated to prevent occlusion and to resolve residual stenosis. A common practice has been to place a retaining structure, such as a rigid or semi-rigid tubular stent, to hold the artery open after angioplasty and retain the dissected plaque material back against the wall of the blood vessel to keep an adequate lumen open for blood flow. The clinical management of dissection or residual narrowing after balloon angioplasty is currently addressed through the development of increasingly complex stent structures. However, there has been substantial clinical evidence of disadvantages with using stents, including body rejection of a large mass of foreign material, and the emplacement of extensive surface area of a stent that may become sites for re-accumulation of plaque re-stenosis due to smooth muscle cell growth and intimal hyperplasia.
In juxtaposition to lesions that may develop significant dissection after balloon angioplasty, a substantial proportion of patients do not sustain major dissections as a result of balloon angioplasty. This seems to depend on several factors, including; the location and morphology of the lesion, and the pressure required to dilate the lesion during balloon angioplasty, but is also to some extent unpredictable. This situation does not require a stent. When post-angioplasty blood vessels show no sign or minimal sign of dissection and are left to heal on their own, i.e., when no stent is implanted, especially in the iliac and femoro-popliteal arteries, the rate of acute re-occlusion is low. The long-term success of balloon angioplasty alone in many cases may produce the same or better long-term results than if a stent was emplaced. Balloon angioplasty without stenting therefore remains one of the most common endovascular procedures and one of the most cost effective.
When it is deemed necessary that a stent is required at a given site of plaque buildup, it is highly desirable to have the ability to fully dilate the stent within the lesion. This is a problem that has been the focus of intensive investigation and is due to the fact that some lesions are so recalcitrant to dilatation, that they cannot be dilated even at very high pressures.
Accordingly, it is deemed highly desirable to dilate plaque material so as to create a smooth post-angioplasty surface without elevated flaps or dissection, and to reduce the need for post-angioplasty stent placement. It is further desirable to provide a method of dilatation that permits better expansion of the lumen, such that if a stent is required, it allows the stent to be fully opened. In cases where local sites of post-angioplasty dissections or non-smooth lumen walls present themselves, it may be desirable to implant a retaining structure other than a stent which offers a minimal surface footprint and exerts low lateral pressures against the post-angioplasty surface.